This invention relates to analog circuits, and more particularly, to RF analog circuits of the type in which several transistors or other amplifying devices are combined to amplify a RF input signal. More specifically, the invention is directed to a technique for controlling the bias levels of selected amplifying devices in response to a degradation in a supply voltage.
The use of amplifiers and other circuits for signal conditioning radio frequency signals is well known in the art. RF analog circuits have been used in radio transceivers, television receivers, CB radios, microwave links, satellite communications systems, local RF networks, and other wireless communication and broadcast applications. A critical component of many amplifiers is the voltage bias circuitry that is used to bias the internal amplifying devices.
It is known that transistors and other amplifying devices typically have an active region in which there is a substantially linear relationship between gate or grid voltage and drain or plate current. The voltage bias circuitry is typically designed to provide a bias level that, when a null signal is applied to the grid, gate or other control electrode, the output current is at a desired quiescent level in about the center of the linear region of the device's active region. This bias condition typically provides optimum performance for the amplifier.
It is also desirable to provide a bias level such that a sufficient voltage is maintained across each amplifying device over all operating conditions, so as to avoid device saturation. If critical amplifying devices enter saturation, the amplifier may no longer operate in the linear range.
One important performance characteristic for many amplifiers is the dynamic range of the amplifier. The dynamic range corresponds to the range from the minimum usable input signal to the maximum usable input signal. The minimum usable input signal is often dictated by the internal noise of the amplifier. The maximum usable input signal is the maximum input signal that the amplifier can accept and amplify without distortion. The maximum usable input signal is typically dependent on a number of factors including the supply voltage used and the gain of the amplifier.
For many applications, such as low power applications, it is desirable to use a relatively low supply voltage. Since the dynamic range of an amplifier is typically dependent on the supply voltage, the dynamic range of an amplifier that is powered by a relatively low supply voltage tends to be less than a similar amplifier that is powered by a higher supply voltage.
In addition, it is also desirable to use a self-contained power source such as a battery or the like for many low power applications. The reason some applications are low power applications is because they are powered by a battery. As is known, however, the voltage provided by batteries tends to degrade over time, especially for alkaline batteries. Accordingly, the dynamic range of an amplifier that is powered by a battery will tend to degrade over time. Moreover, if the supply voltage degrades beyond some critical point, some or all of the amplifying devices may enter saturation and the amplifier may cease to operate in a linear mode.
One approach for maintaining the functionality and performance of an amplifier when using a low and/or degrading power supply is to optimize the bias levels under worst case conditions. For example, the bias levels may be set so that after the supply voltage degrades over time, a sufficient voltage is still maintained across each amplifying device to prevent device saturation. A limitation of this approach is, of course, that the performance of the amplifier is typically less than optimal under nominal conditions. Further, the design constraints imposed by designing an amplifier that uses fixed bias levels to maintain satisfactory performance over all operating conditions can increase the complexity of the design.
What would be desirable, therefore, is an amplifier that has a compensation circuit that dynamically compensates selected bias levels within the amplifier to maintain operation and performance over a relatively wide range of supply voltages. This may increase the performance of the amplifier under all operating conditions, and may significantly simplify the amplifier design.